222 PALAEONTOLOGY OF NEW YORK. 



A left valve has a length nl" "il iiiiii., heiglit 37 iiiiii., Iiinw-line about 

 42 mill. 



Specimens of this species usually present a vei'j rough and irreuulai- aspect 

 Ironi the character of the shell, with its strongly lamellose structure and tlic 

 coarse matrix in which it is usually Ibmid. The specimen figured is a cast of 

 the interior, and is distinguished by the broad posterior end and aluupt narrowing 

 toward the front, the obtusely sub-angular lunbo, and the large anterior end. 

 In a crushed .specimen retaining both valves, the right valve is almost equally 

 convex with the left; the surface is less lamellose and presents only gentle 

 undulations. 



Formation and locality. In a coarse sandstone of the uj)per part of the 

 Chemung group, on tlic road from Olean, X. Y., to Sniethport, Pa. 



Leptodesma Biton, n. sp. 



I'l.AlE X(J1. vm i. 



Shell large, sub-rhomboidal ; body elongate-ovate, making an angle of about 

 45° with the hinge-line ; height nearly two-thirds of the length ; ante-byssal 

 margin oblique, slightly curving into a long, shallow sinus, which impresses 

 the margin for more than half the length of the valve; basal and posterior 

 margins broadly rounded. 



Left valve regularly and gently convex in the lower part, becoming more 

 convex and gibbous above the middle. Kight valve uidxnowii. 



Hinge-line straight, greater than the heiglit of \\\v \al\c. 



Beak sub-anterior, directed forward, but little elevated above the liinge- 

 line. Umbonal region al)ruptly gibljous, subtending an acute angle. 



Anterior end produced, narrow, acute at the extremity, limite<l by an 

 oblique, shallow byssal depression. Wing large, joining the bod}- at the 

 posterior end, not distinctly separated : margin slightl}- ol)lique below, gently 

 concave above the middle, and curving slightly outward just below the car- 

 dinal line. 



Test thill ; cast marked by fine concentric striae of growth, which, at 



